Choosing the Right Water Heater Size
Selecting the correct water heater size prevents two common problems: running out of hot water during peak usage and overpaying for a unit that is larger than necessary. The key metric for tank water heaters is the First Hour Rating (FHR), which measures how many gallons of hot water the unit can deliver in the first hour of use starting with a full tank. Your FHR requirement depends on how many people live in your home and how much hot water they use simultaneously.
For a household of 2-3 people with average usage, a 50-gallon tank with an FHR of 60+ gallons is typically sufficient. Larger families of 4-5 people should look at 65-80 gallon tanks. Homes with multiple bathrooms may experience higher simultaneous demand even with fewer occupants, so this calculator adds capacity for each bathroom beyond two.
Tankless (on-demand) water heaters are sized by flow rate in gallons per minute (GPM) rather than tank capacity. A typical shower uses 2.0-2.5 GPM, a dishwasher 1.5 GPM, and a washing machine 2.0-4.0 GPM. You need a tankless unit rated for the total GPM of all fixtures that might run simultaneously during peak usage.
Common Water Heater Sizes and Typical FHR
| Tank Size | Typical FHR (Gas) | Typical FHR (Electric) | Best For |
|---|---|---|---|
| 30 gal | 50-60 gal | 35-45 gal | 1 person |
| 40 gal | 60-70 gal | 45-55 gal | 1-2 people |
| 50 gal | 70-85 gal | 55-65 gal | 2-3 people |
| 65 gal | 85-100 gal | 65-80 gal | 3-4 people |
| 80 gal | 100-120 gal | 80-95 gal | 4-5 people |
| 100 gal | 120+ gal | 95+ gal | 5+ people |